A. Recombinant DNA Technology
With the advent of recombinant DNA technology, the controlled microbial production of an enormous variety of useful polypeptides has become possible, putting within reach the microbially directed manufacture of hormones, enzymes, antibodies, and vaccines useful against a wide variety of diseases. Many mammalian polypeptides, such as human growth hormone and leukocyte interferons, have already been produced by various microorganisms.
One basic element of recombinant DNA technology is the plasmid, an extrachromosomal loop of double-stranded DNA found in bacteria oftentimes in multiple copies per cell. Included in the information encoded in the plasmid DNA is that required to reproduce the plasmid in daughter cells (i.e., a "replicon") and ordinarily, one or more selection characteristics, such as resistance to antibiotics, which permit clones of the host cell containing the plasmid of interest to be recognized and preferentially grown in selective media. The utility of such bacterial plasmids lies in the fact that they can be specifically cleaved by one or another restriction endonuclease or "restriction enzyme", each of which recognizes a different site on the plasmidic DNA. Heterologous genes or gene fragments may be inserted into the plasmid by endwise joining at the cleavage site or at reconstructed ends adjacent to the cleavage site. (As used herein, the term "heterologous" refers to a gene not ordinarily found in, or a polypeptide sequence ordinarily not produced by, a given microorganism, whereas the term "homologous" refers to a gene or polypeptide which is found in, or produced by the corresponding wild-type microorganism.) Thus formed are so-called replicable expression vehicles.
DNA recombination is performed outside the microorganism, and the resulting "recombinant" plasmid can be introduced into microorganisms by a process known as transformation and large quantities of the heterologous gene-containing recombinant plasmid are obtained by growing the transformant. Moreover, where the gene is properly inserted with reference to portions of the plasmid which govern the transcription and translation of the encoding DNA, the resulting plasmid can be used to actually produce the polypeptide sequence for which the inserted gene codes, a process referred to as expression. Plasmids which express a (heterologous) gene are referred to as replicable expression vehicles.
Expression is initiated in a DNA region known as the promotor. In some cases, as in the lac and trp systems discussed infra, promotor regions are overlapped by "operator" regions to form a combined promotor-operator. Operators are DNA sequences which are recognized by so-called repressor proteins which serve to regulate the frequency of transcription initiation from a particular promoter. In the trancription phase of expression, RNA polymerase recognizes certain sequences in and binds to the promoter DNA. The binding interaction causes an unwinding of the DNA in this region, exposing the DNA as a template for synthesis of messenger RNA. The messenger RNA serves as a template for ribosomes which bind to the messenger RNA and translate the mRNA into a polypeptide chain having the amino acid sequence for which the RNA/DNA codes. Each amino acid is encoded by a nucleotide triplet or "codon" which collectively make up the "structural gene", i.e., that part of the DNA sequence which encodes the amino acid sequence of the expressed polypeptide product.
After binding to the promoter, RNA polymerase initiates the transcription of DNA encoding a ribosome binding site including a translation initiation or "start" signal (ordinarily ATG, which in the resulting messenger RNA becomes AUG), followed by DNA sequences encoding the structural gene itself. So-called translational stop codons are transcribed at the end of the structural gene whereafter the polymerase may form an additional sequence of messenger RNA which, because of the presence of the translational stop signal, will remain untranslated by the ribosomes. Ribosomes bind to the binding site provided on the messenger RNA, in bacteria ordinarily as the mRNA is being formed, and direct subsequently the production of the encoded polypeptide, beginning at the translation start signal and ending at the previously mentioned stop signal(s). The resulting product may be obtained by lysing the host cell and recovering the product by appropriate purification from other bacterial proteins. Polypeptides expressed through the use of recombinant DNA technology may be entirely heterologous, functional proteins, as in the case of the direct expression of human growth hormone, or alternatively may comprise a bioactive heterologous polypeptide portion and, fused thereto, a portion of the amino acid sequence of a homologous polypeptide, as in the case of the production of intermediates for somatostatin and the components of human insulin. In the latter cases, for example, the fused homologous polypeptide comprised a portion of the amino acid sequence for beta galactosidase. In those cases, the intended bioactive product is rendered bioinactive within the fused, homologous/heterologous polypeptide until it is cleaved in an extracellular environment. Fusion proteins like those just mentioned can be designed so as to permit highly specific cleavage of the precusor protein from the intended product, as by the action of cyanogen bromide on methionine, or alternatively by enzymatic cleavage. See, eg., G.B. Patent Publication No. 2 007 676 A.
If recombinant DNA technology is to fully sustain its promise, systems must be devised which optimize expression of gene inserts, so that the intended polypeptide products can be made available in controlled environments and in high yields.